1. Field of the Invention
The present invention pertains to methods and apparatus for accurately determining the frequency of signals in the Gigahertz range. More specifically, this invention relates to methods and apparatus for determining the residual count of a prescaler of the type frequently employed in conjunction with a direct counter.
2. Description of the Prior Art
Digital methods and apparatus for measuring high frequency signals (i.e. above 500 MHz) are hindered by the inability of present day counters to accept and produce an accurate indication of frequency. To overcome this inherent problem, high speed discrete logic, prescaler operations and down converting (or heterodyning) of the input signal to an intermediate frequency have been employed to convert the frequency of the input signal to one compatible with the capabilities of the direct counter.
Each of the aforementioned techniques possesses unique advantages and suffers peculiar disabilities with respect to the measurement of signal frequencies. Systems employing high speed discrete logic often utilize a considerable number of components, commonly rendering such systems undesirable from either a cost or a reliability viewpoint. The mixing of an unknown frequency with a known frequency in a typical heterodyning operation possesses inherent disadvantages occasioned by drift, temperature instability and the like.
Prescaling operations reduce the frequency of the unknown input signal by a known factor. The reduced frequency signal may then be applied to a direct counter which counts the number of cycles of the "prescaled" signal. The actual frequency of the input signal may then be determined by multiplying the measured count by the divide ratio of the prescaler. The prescaler apparatus often comprises a chain of bistable logic elements, certain of the outputs of which are gated through conventional logic elements to produce an output comprising a single pulse representative of a preselected number of input pulses (after proper condition of the input signal, if analog). Examples of prescaling apparatus include the SP8665D, SP8666B and SP8667B decade counters manufactured by Plessey Semiconductors of Irvine, Calif. Each of these UHF counters produces a single output pulse for every 10 pulses of the input signal.
The accuracy of frequency measurement wherein a prescaler operation is employed is often hindered by the existence of "residual" counts in the prescaler. Such residue is occasioned by the fact that a finite segment of the unknown signal comprising a number of pulses is gated into the prescaler and, unless such segment contains pulses comprising a whole number multiple of the prescaler divide ratio, only a portion of the frequency information applied to the prescaler will be passed to the direct counter. Manufacturing and operational considerations relating to reliability and stability have precluded the manufacture of prescalers with parallel outputs for use above about 700 MHz, leaving such delay-inducing techniques as extending the duration of the gate to the time equivalent of the predetermined prescaler divide ratio.